The present invention relates to implantable pacemakers, and more particularly to an implantable pacemaker wherein the paced AV delay is automatically adjusted based on the timing of the atrial-evoked potential.
The paced AV delay of an implantable pacemaker is the time interval between application of an electrical stimulus to the atrium (A-pulse) and the application of an electrical stimulus to the ventricle (V-pulse). The AV delay represents the best estimate of the nodal conduction time in the patient's heart as a stimulus travels from the atria to the ventricles. That is, the AV delay represents an estimate of the optimum time between atrial depolarization and ventricular depolarization.
Early demand pacemakers employed a timer circuit that generated a fixed AV interval (AVI) that was triggered by the generation of an A-pulse or by the sensing of atrial depolarization (P-wave). If ventricular depolarization (an R-wave) was not sensed before the termination of the AVI, a ventricular stimulus (V-pulse) was generated. In this manner, the fixed AVI of the pacemaker represented the maximum allowable time between depolarization of the atria and depolarization of the ventricles.
With the advent of programmable demand pacemakers, it became advantageous to make the AVI of the pacemaker a programmable value. This is because the optimum AV delay, i.e., the time delay between atrial and ventricular depolarization, varies greatly between different patients. Further, the optimum AV delay for a given patient may vary widely depending upon various factors, including whether the patient is being administered drugs, the heart rate (e.g., level of exercise) of the patient, etc. Thus, being able to set the AVI of the pacemaker to a desired value for a particular patient at a particular time provided a significant benefit to the pacemaker patient.
However, experience soon indicated that the optimum AVI of the pacemaker for pacing was different than the optimum AVI for sensing. That is, it was found that if an A-pulse was delivered to the atrium, the optimum AVI should be somewhat longer, e.g. 25 milliseconds, than if the AVI was triggered by sensing a P-wave. Hence, it is known in the pacemaker art to generate one AVI in response to generating an A-pulse, and another (shorter) AVI in response to sensing a P-wave. This difference in optimum values has been attributed to intra-atrial conduction delays. That is, depending upon the particular location at which an atrial stimulus (A-pulse) is applied to the atrial tissue (typically in the right atrial appendage), it takes a finite time for this stimulus to reach the primary inter-atrial conduction pathways (internodal tracts) of the heart. Once the stimulus does reach the inter-atrial conduction paths, rapid conduction occurs, resulting in depolarization of the right atrium followed by depolarization of the left atrium. See, e.g., Ausbel et al., "Interatrial Conduction During Cardiac Pacing," PACE, Vol. 9, pp. 1026-1031 (Nov/Dec 1986, Part II); Wish et al., "Importance of Left Atrial Timing in the Programming of Dual-Chamber Pacemakers," Am. J. of Cardiol., Vol. 60, pp. 556-571 (Sept. 1987); Alt et al., "Different Beneficial AV Intervals with DDD Pacing After Sensed or Paced Atrial Events," J. of Electrophys., Vol. 1, No. 3, pp. 250-256 (1987); and Catania, et al., "AV Delay Latency Compensation," J. of Electrophys., Vol. 1, No. 3, pp. 242-249 (1987). In other words, when the pacemaker applies an A-pulse to the right atrium, such application does not cause instantaneous depolarization of the atria. Rather, the applied stimulus must be transferred through available conduction paths within the right atrium, and between the right and left atria, before atrial depolarization can occur. In contrast, when the pacemaker senses a P-wave, depolarization of the atria is already in process. Thus, it is known in the art for pacemakers to incorporate an automatic shortening of the programmed AVI after a sensed atrial event (P-wave). This automatic shortening attempts to make the effective AV delay (the time from the atrial depolarization to the ventricular depolarization) substantially the same regardless of whether the pacemaker is pacing or sensing. Unfortunately, however, such shortening of the AVI adds to the complexity of the pacemaker circuits, and may not provide the correct amount of shortening for a given patient at all times. This is because, as indicated, the inter-atrial and/or intra-atrial conduction times (latency time) vary greatly from patient to patient, and also vary a great deal for a particular patient at different times. Hence, what is needed is a simple means for automatically adjusting the programmed AVI of a pacemaker to include the latency time of a particular patient, whatever that latency time may be. The present invention advantageously addresses this and other needs.